In process measurements technology, or in industrial measurements technology, for measuring the conductivity of a liquid, frequently conductivity sensors are used, which work according to an inductive or a conductive measuring principle.
Known from EP 990 894 B1, for example, is a conductive conductivity sensor, which has at least two electrodes, which, for measuring, are immersed in a measured medium. For determining the electrical conductivity of the measured medium, the resistance or conductance of the electrode measuring path in the measured medium is determined. In the case of a known cell constant, the conductivity of the measured medium can be ascertained therefrom. For measuring the conductivity of a measured liquid by means of a conductive conductivity sensor, it is absolutely required to bring at least two electrodes in contact with the measured liquid.
Inductive conductivity sensors comprise a transmitting coil and a receiving coil, which are, as a rule, embodied as toroidal coils, and which surround a traversing opening chargeable with the measured liquid, so that, upon exciting the transmitting coil, a closed electrical current path can form extending within the medium. The path passes through the transmitting coil and the receiving coil. By evaluating the electrical current- or voltage signal of the receiving coil in response to the signal of the transmitting coil, consequently, the conductivity of the measured liquid can be ascertained. Examples of inductive conductivity sensors are known, for example, from DE 197 47 273 B4, EP 999 441 B1 or DE 4116468 A1. These conductivity sensors are embodied as probes, which, for measuring conductivity, are immersed into the measured liquid, so that measured liquid flows around the two toroidal coils.
Pharmaceutical, chemical, biological, biochemical or biotech processes are performed, in increasing measure, in single-use, process containers (also referred to as ‘disposables’, or, in the biotech field, for instance, ‘disposable bioreactors’). Such single-use, process containers can be, for example, flexible containers, e.g. bags, tubes or fermenters, respectively, bioreactors. Bioreactors, or fermenters, possess, frequently, feed- and drain lines, which can be embodied, for example, as tubes. In the feed- and drain lines, also fixed tubular pieces can be applied. After terminating a process the single-use, process container can be disposed of. In this way, complex cleaning- and sterilization methods can be avoided. Especially, through the use of single-use, containers, the risk of cross contamination is avoided and, therewith, bio- and process safety increased. Single-use, process containers are, as a rule, made of synthetic material, for instance, plastic.
The processes running in the single-use, process containers are sealed relative to the environment. Before introducing process media into the single-use, process containers, such must, as a rule, be sterilized. For this purpose in biochemical, biological, biotechnological and pharmaceutical applications, frequently, gamma radiation is used. Also, during the running of a process in a single-use fermenter or single-use reactor, the penetration of germs from the environment into the interior of the container must be avoided, in order not to degrade or corrupt the process.
In order to monitor the processes, it can be necessary to measure physical or chemical, measured variables of the media contained in the container. Measured variables to be monitored can include, besides electrical conductivity, for example, temperature, pH-value, cell density, optical transmission or concentration of a chemical substance, for example, a certain kind of ion or a certain element or a certain compound. For assuring and maintaining sterility within the process container, it is especially desirable, to measure these measured variables with contactless methods.
Known from DE 37 18 111 C2 is an arrangement for contactless, inductive measuring of the conductivity of a measured liquid, in the case of which the measured liquid flows through a line, for example, a hose or a pipe, which has two liquid paths, so that a liquid loop is formed. A first, toroidal coil serving as exciter coil surrounds the first liquid path of the liquid loop, a second, toroidal coil serving as receiving coil surrounds the second liquid path of the liquid loop, so that, upon exciting of the exciter coil, a closed electrical current path forms within the measured liquid flowing in the liquid loop. The path passes through the exciter coil and the receiving coil, so that an electrical current, or a voltage, is induced in the receiving coil, based on which the conductivity of the measured liquid can be ascertained.
Known from DE 198 23 836 C2 is another arrangement for contactless, inductive measuring of the conductivity of a measured liquid, which is supposed to be suitable for application in the case of single-use, process containers. This arrangement has only a single toroidal coil, which surrounds a pipeline flowed through by the measured liquid. The toroidal coil can be excited to produce a time variable, magnetic field, which induces an electrical current within the liquid flowing through the line. In contrast to the arrangement described in DE 37 18 111 C2, however, no electrical current induced in the liquid is measured, but, instead the power loss of the measuring arrangement brought about by the electrical current flow in the liquid and the ohmic resistance of the liquid and therefrom the conductivity of the measured liquid is ascertained.
Both the arrangement described in DE 37 18 111 C2 as well as also the arrangement known from DE 198 23 836 C2 are, indeed, suitable for contactless measurement. However, they can only be applied in connection with a tubular line carrying the measured liquid and extending axially through the toroidal coils. The arrangement according to DE 37 18 111 C2 even requires a line embodied in special manner to form a liquid loop, in order to have a closed electrical current path through the measured liquid. They are, consequently, for example, suitable only for measuring conductivity in specially dimensioned lines. Thus, they can not be applied directly, for example, in the bag fermenters installed, frequently, in single-use, process technology, but, instead only in a suitably dimensioned supply or drain line.
Another apparatus for contactless measurement of electrical conductivity in a measured liquid is known from DE 199 48 465 A1. The apparatus is based on the principle of known eddy current methods, as they are applied, for example, in materials testing. The apparatus has a cylindrical exciter coil for producing a magnetic, alternating field in the measured liquid, a cylindrical receiving coil arranged within the windings of the cylindrical exciter coil for measuring a magnetic field resulting from the alternating magnetic field and the magnetic field of electrical currents, which are induced in the liquid due to the alternating magnetic field and whose magnetic field is directed counter to the alternating magnetic field, and a corresponding measurement circuit for ascertaining the conductivity of the liquid from the measured resulting magnetic field. This apparatus can be arranged outside of a container of any geometry, and measure contactlessly through a non electrically conductive wall of the container the conductivity of a contained measured liquid therein. Since not only the magnetic field induced in the measured liquid, but also the magnetic field of the exciter coil acts on the receiving coil, the electrical signal of the receiving coil will always be relatively large. Especially, the part of the magnetic field from the exciter coil predominates significantly compared to the part of the electrical signal of the receiving coil due to the conductivity of the measured liquid. This degrades the accuracy of measurement of the apparatus, especially in the case of small conductivities of the measured liquid.